Nuclear magnetic resonance (“NMR”) is one of the most widely used analytical techniques in chemistry and structural biology. It is an essential tool in drug discovery and design and has a tremendous variety of applications in chemistry, biochemistry and engineering.
A typical 600 MHz NMR magnet includes several miles of superconducting wire to form a nearly perfect solenoid close to the center of the cryostat. At liquid helium temperature the solenoid supports an extremely high electrical current that creates a large and uniform magnetic field. A sample tube rides a stream of air from the top of the magnet to roughly the center of the magnet. The bottom of the sample tube is positioned within the top of the “probehead” that bears a small coil to induce and detect transitions between the spin states of protons, carbon-13, nitrogen-15, or other nuclei of interest. A brief, intense radiofrequency pulse (˜10 μs) at 600 MHz generated by the probehead is needed to excite proton spins; other nuclei can be excited at other frequencies. A short time after the pulse, the same coil is used to detect the decay of the spins back to their equilibrium state. The voltage induced in the coil is amplified and transformed to a display of peaks in a spectrum. The dispersion of peaks and their individual characteristics in a spectrum provide rapid feedback to a chemist regarding chemical composition, relative concentration, and purity in a sample. More extended analysis is required to determine details of three-dimensional structure, dynamics, and the stability of complexes.
A typical NMR spectrometer process involves placing a liquid sample into a long (18–25 cm), thin (4.97 mm outer diameter), high-precision glass sample tube that is then suspended vertically inside a close-fitting core in the NMR's intense magnetic field. The air gap between the glass sample tube and the surrounding core is typically only 0.300 mm. The tube with sample is then usually spun axially, typically at 20 Hz, to provide an average view of the sample to the NMR detectors and lessen the effect of imperfect magnetic field homogeneity. While the NMR measures the spectra of the sample, the magnetic lines of force are unfortunately distorted by the glass sample tube because all undoped glasses have bulk magnetic susceptibilities that are not well matched to the bulk magnetic susceptibilities of chemical and biological solvents normally used in NMR experiments. These discontinuities introduce unwanted spectral artifacts that should be minimized, if possible, by the skilled researcher. To reduce this problem, the glass tube walls are made as thin as practicable (typically ranging downward from 0.38–0.22 mm, with special case applications of 0.19 mm known) to reduce the amount of glass present, constrained by the need to avoid breakage and losing the sample, especially inside the NMR magnet. Spectral artifacts can become severe unless the tube walls are extremely uniform in thickness, precisely cylindrical and almost perfectly straight (this also helps to avoid potentially expensive damage to the instrumentation). High precision glass sample tubes are thus a substantial part of the cost of operating an NMR spectrometer.